1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube capable of preventing degradation of color purity of a panel by optimizing a screen transmittance of the panel and improving brightness uniformity.
2. Discussion of the Related Art
Generally, a cathode ray tube is a device for converting an electric signal into an electric beam and implementing an image by emitting the electron beam onto a phosphor screen. The device produces excellent display quality for a low price, and accordingly it is widely used.
As shown in FIG. 1, the cathode ray tube according to a related art includes a panel 101 which is a front glass, a funnel 102 which is a rear glass for forming a vacuous space when combined with the panel 101, a phosphor screen 113 for functioning as a luminescent material while being coated on an inner surface of the panel 101, an electron gun 106 arranged in a neck of the funnel 102 for emitting an electron beam 105, a deflection yoke 107 for deflecting the electron beam 105 onto the phosphor screen 113 being mounted on an outer circumferential surface of the funnel 102, a shadow mask 108 installed at a predetermined distance from the fluorescent surface 113, a mask frame 109 for fixing/supporting the shadow mask 108, and an inner shield 110 installed inside the funnel 102 for preventing the color purity from being degraded by external magnetic fields.
In operation, the electron beam 105 generated from the electron gun 106 is deflected by the deflection yoke 107, and lands on the phosphor screen 113, which is formed on an inner surface of the panel 101, after passing through a plurality of electron beam passage holes formed in the shadow mask 108. Then, the corresponding green, blue and red phosphors disposed on the phosphor screen 113 are radiated by the electron beam 105, thereby displaying a color image.
Herein, a brightness difference occurs according to a transmittance of the shadow mask 108, a transmittance of the phosphor screen 113 (hereinafter, it is referred to as a ‘screen transmittance’) and a transmittance of the panel 101 (hereinafter, it is referred to as a ‘glass transmittance’). Here, the transmittance of the shadow mask 108 is about 14–19%, the screen transmittance is about 45–60%, and the glass transmittance is about 70–80%. These three kinds of transmittance decrease gradually along from a center portion of the panel 101 to a peripheral portion. Therefore, such differences in transmittance of respective portions of the panel 101 degrade brightness uniformity of the whole surface of the panel 101.
Also, as shown in FIGS. 2A and 2B, as a curved panel having an outer surface with a small radius of curvature is changed to a flat panel having an outer surface with an almost infinite radius of curvature, a wedge ratio, which is a thickness ratio between the center portion of the panel 101 and the peripheral portion of the panel 101, increases. Therefore, as the difference of the glass transmittances between the center portion and the peripheral portion of the panel 101 increases the brightness uniformity of the screen decreases.
In order to improve the brightness uniformity of the cathode ray tube, a glass having a high optical transmittance can be applied to the panel 101 for increasing the glass transmittance of the peripheral portion of the panel 101. However, doing so would deteriorate contrast characteristics including contrast ratio. Therefore, to solve the problem of degraded image contrast, a method of coating colorant or attaching a film containing the colorant on an outer surface of the panel glass may be used. However, it requires an additional coating process, which is generally not necessary for a non-flat type color cathode ray tube. Accordingly, it raises such problems as additional number of parts, additional production cost, difficulties caused by additional production processes and a reduction in yield.
As another method for simultaneously improving the brightness uniformity and contrast characteristics, a tinted glass or a dark-tinted panel glass can be applied on the panel without performing such processes as coating or the like. As shown in Table 1 below, if the tinted glass or the dark-tinted panel glass is applied, the transmittance rapidly decreases along from the center portion to the peripheral portions of the panel. This deteriorates brightness uniformity of the center and peripheral portions. FIG. 3 shows the deterioration of brightness uniformity described above, as the brightness of the center portion of the panel is high and the brightness of the peripheral portion is low, resulting in a ‘white ball phenomenon’ in that a white spherical shape appears in a center of a screen.
TABLE 1Glass Transmittance (%)Panel GlassCenter portionDoming portionPeripheral portionCleared807470Tinted513527Dark tinted402418
Table 1 compares glass transmittances at the respective portions of a panel with a tinted glass having a wedge ratio of 200%, a panel with a dark-tinted glass, and a panel with a clear glass without using a tinted or dark-tinted glass. In Table 1, the doming portion is a region positioned between the center portion and the peripheral portion of the panel and affected by a doming effect in which a landing position where the electron beam is landed on the phosphor screen is displaced by heat expansion of the shadow mask caused by impingement of the electron beam.
On the other hand, a method of reducing the wedge ratio is considered. That is, the thickness of the peripheral portion of the panel is reduced to increase the optical transmittance of the peripheral portion of the panel, thereby to improving the brightness uniformity of the whole panel. Herein, by reducing the wedge ratio, the inner surface of the panel becomes flat, which means, a radius of curvature of the inner surface of the panel is increased. Also, a radius curvature of the shadow mask having a dome shape and maintaining a certain distance from the inner surface of the panel must be changed in accordance with the change in the curvature radius of the inner surface of the panel.
However, the radius of curvature of the shadow mask is a main factor determining a howling characteristic according to a structural stiffness, an internal impact resistance, and an external impact resistance of the shadow mask. Thus, if the radius of curvature of the shadow mask increases in accordance with the inner surface of the panel, the mechanical strength of the shadow mask decreases, and the shadow mask is easily deformed during the manufacturing processes.
Therefore, to improve brightness uniformity of the display, there is a limitation to reducing the wedge ratio of the panel, and a more efficient method for improving brightness uniformity is required.